High speed Diesel engines or simply HSD engines, usually used in passenger and light load cars, have as operational characteristics: high speed, around 4,500 rpm, peak combustion pressures (PCP) over 9 or 12 MPa, usually 14 MPa, and maximum piston temperatures of about 330 degrees Celsius. These engines should be equipped with pistons which are highly resistant to high thermal and mechanical efforts and, besides this, it is desirable that the pistons in these engines have low operational noise, reduced weight and low manufacturing cost.
In order to meet the operational requirements of the HSD engines, nowadays there are several piston constructions, resulting from different technical approaches, as discussed below.
In a first technical approach, already known from the German Patent Applications DE 26 07 624, DE 36 39 806, DE 31 19 113, DE 34 44 406 and DE 40 10 474, the pistons are constructed in a light and relatively maleable metal, consequently having low weight and relative elasticity in order to absorb the impacts against the cylinder, which is advantageous in terms of reducing the operational noise. On the other hand, the piston constructions described in said applications have in common thick walls and local reinforcements in order to increase their mechanical resistance. In general, the thickening of walls and the local reinforcements are provided in regions of very high efforts, such as the region of the piston rings (ring-carriers), around the edge of the combustion chamber (fiber collars) and in the wrist pin bosses for supporting the wrist pin articulating the connecting rod, by inserting bushings therebetween. Wall thickening and reinforcement insertion is a common practice to impart more resistance to the light metal pistons, in order to overcome resistance deficiencies during operation and it is not an advantageous technical approach. The manufacture of pistons according to this approach, besides being technically difficult, is costly and many times does not result in a piston with long useful life or with a sufficiently low operational noise so that the engine may satisfy the present and future environmental requirements because, as the walls are thickened, specially in the parts contacting the cylinder, the resistance gain occurs to the detriment of the elasticity, and the insertion of local reinforcements increases the weight and brings risks to the piston reliability, mainly due to the integral connection with the reinforcement.
In order to increase the resistance of the piston, without thickening the walls or applying local reinforcements, it is possible to construct said piston with more resistant materials, such as iron or steel but, again, the weight would relevantly increase, while the elasticity would decrease, impairing the efficiency of the engine and increasing its noise level, respectively.
Besides these approaches, more recently, Patent Application published with the No. WO 95/04880 has suggested the combination of a more resistant aluminum alloy with a certain piston construction, which would resist to the characteristic efforts of the HSD engines, being provided with thinner walls and without using local reinforcements. According to the concept described in said patent application, in order to reduce the piston weight, there is provided a certain reduction in the thickness of the skirt walls on the pressure and counter pressure sides of the piston, both radially and circumferentially, taking into account the dimensional limitations for shaping other portions of the piston, such as compression height and wrist pin bosses dimensions, only to mention some of said limitations.
Although it can resist to efforts, a piston constructed according to this last approach would still be considered heavy and with a high operational noise, regarding the highest requirements of the car manufacturers, since the parts of said piston, which are the object of the modifications proposed in said patent application, specially the piston and combustion chamber diameters, compression height and those dimensions for the wrist pin bosses, offer very little possibility to reduce their dimensions, because they have to be worked so as to adjust to the ranges of the basic dimensional specifications given by the engine manufacturers (engine baseline).
The piston overweight leads to engine power loss, since a significant part of the energy generated in the combustion ceases to be transferred to move the vehicle, said energy being internally consumed to draw the piston itself. An expressive number to evaluate how much the piston weight represents to the engine is the rate that comprises the piston weight for each kilowatt of the engine power, this rate, even for the HSD engine pistons considered lighter by the prior art, ranging around 24 to 26 grams per kilowatt of power for each engine cylinder. The low elasticity in the piston portions that contact the cylinder during its stroke, also known as pressure and counter pressure sides, results in an undesirable operational noise, generated by the piston strikes against the cylinder.